The selective reduction of carbonyl compounds to the corresponding alcohols in the course of which only the reaction of the C.dbd.O finction is observed, is an important task in the field of organic chemistry. Until now, there were exclusively used hydride reducing agents, such as lithium aluminum hydride LiAlH.sub.4, sodium borohydride NaBH.sub.4, or sodium dihydroxybis(2-methoxyethoxy)aluminate (SDMA) of formula NaAlH.sub.2 (OCH.sub.2 CH.sub.2 OCH.sub.3).sub.2, the two latter reagents being of limited value for the reduction of esters and lactones. All the above-mentioned reagents are employed in stoichiometric amounts and show the disadvantage of releasing hydrogen in the course of the reaction or, when entering into contact with humidity, of leading to explosion risks and requiring the inertization of the reactors used. Furthermore, the use of these reagents is costly as they are required in stoichiometric amounts. Thus, there is a continous search for other systems which are more economic and easier to use.
Several publications describe the use of silanes as reducing agents for carbonyl substrates, together with a metal catalyst. A preferred silane for this type of reductions is polymethylhydrosiloxane or PMHS, according to the general formula ##STR1##
U.S. Pat. No. 3,061,424 to Nitzsche and Wick describes the reduction of aldehydes and ketones with PMHS and a salt of mercury, iron, copper, titanium, nickel, zirconium, aluminum, zinc, lead, cadmium and, as the preferred embodiment, tin. This reductive system requires activation by a proton source, without which the reaction does not proceed. However, the system is not effective for the reduction of esters and lactones.
U.S. Pat. No. 5,220,020 to Buchwald et al. describes a method for the preparation of alcohols by the reduction of carbonyl compounds using a system composed of a silane reducing agent and a metal catalyst of formula M(L)(L.sup.I)(L.sup.II) to M(L)(L.sup.I)(L.sup.II)(L.sup.III)(L.sup.IV)(L.sup.V), in which M is a metal belonging to any of groups 3, 4, 5, or 6 of the periodical table, a lanthanide or an actinide, whereas (L.sup.I) to (L.sup.V) represent hydrogen, an alkyl group, an aryl group, a silyl group, a halogen atom, or a --OR, --SR or --NR(R') group, R and R' being hydrogen, an alkyl or an aryl group. Amongst the preferred catalysts, the cited patent mentions titane (IV) isopropylate or ethylate or trichlorotitane (IV) isopropylate. Such a system is said to be appropriate for the reduction of esters, lactones, amides or imines. More recently, Breedon and Lawrence (Synlett., 1994, 833) and Reding and Buchwald (J. Org. Chem., 1995, 60, 7884) have described a similar process, namely the use of non-activated titane tetraalkoxydes as catalysts for the reduction of esters by PMHS. The method described in those three mentioned references requires the use of large amounts, at least 25 mole % with respect to the substrate, of catalyst. Barr, Berk and Buchwald (J. Org. Chem., 1994, 59, 4323) have shown that the complex Cp.sub.2 TiCl.sub.2, when reduced by butyllithium or ethylmagnesium bromide, could catalyze the reduction of esters into the corresponding alcohols with good yields, but this technique requires reagents which are expensive and difficult to use in a large scale, as is the case in industrial organic synthesis.
As closest prior art, there should be cited the international application WO 96/12694 of the applicant, describing the reduction of aldehydes, ketones, esters and lactones by a reductive system composed of silanes and a metal hydride, leading to the corresponding alcohols with good yields. This systems requires only very low amounts of catalyst, i.e. the metal hydride, in the order of 1 mol % with respect to the substrate. The hydride is formed by the reaction of a salt of the respective metal with an appropriate reducing agent, preferably NaBH.sub.4. Besides zinc salts, cobalt, manganese and iron salts are used as precursors for the generation of metal hydrides. According to another preferred embodiment, PMHS is used as silane reducing agent.